The present invention relates to a method and apparatus for cooling, optionally liquefying, a hydrocarbon stream, particularly but not exclusively natural gas. In other aspects, the present invention relates to a method and apparatus for cooling a mixed refrigerant stream.
Several methods of liquefying a natural gas stream, thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure.
U.S. Pat. No. 4,404,008 describes a method for cooling and liquefying a methane-rich gas stream which is first heat exchanged against a single component refrigerant, such as propane, and then a multi-component refrigerant, such as lower hydrocarbons. The single component refrigerant is also used to cool the multi-component refrigerant subsequent to the multi-component refrigerant's compression. The arrangement shown in U.S. Pat. No. 4,404,008 is now considered to be a common methodology for liquefying natural gas where the multi-component refrigerant is pre-cooled by the single component refrigerant by passing them through the same first heat exchanger.
An object of U.S. Pat. No. 4,404,008 is to shift refrigeration load from the multi-component refrigeration cycle to the single component refrigeration cycle. This is achieved by utilising inter-stage cooling of the multi-component refrigerant cycle.
However, control of a multi-component pre-cooling refrigeration cycle can be unsatisfactory using existing methods.